A number of systems have been proposed in which a printed medium is imbued with an ability to support user interactions with a computer system. Such systems typically rely on coded data placed on or in the medium, ideally in inconspicuous or invisible form. The interactions are then typically mediated by a device capable of sensing and decoding the coded data and communicating the interactions to the computer system. The coded data can consist of one or more discrete barcodes, with each barcode identifying a hyperlink to related information. For example, the barcode can be a UPC barcode as described in U.S. Pat. No. 5,978,773 “System and method for using an ordinary article of commerce to access a remote computer” (F. C. Hudetz et al); it can also consist of a coded coordinate grid which allows the sensing device to generate position information relative to the medium, as variously described in U.S. Pat. No. 5,051,736 “Optical stylus and passive digitizing tablet data input system” (W. E. Bennett et al), U.S. Pat. No. 5,477,012 “Optical position determination” (O. F. Sekendur), U.S. Pat. No. 6,330,976 “Marking medium area with encoded identifier for producing action through network” (M. Dymetman et al), and U.S. Pat. No. 6,502,756 “Recording of information” (C. Faahraeus), as well as in the present applicant's granted and co-pending applications listed above.
As described in the present applicant's U.S. Pat. No. 6,457,883, it is advantageous to include unique identifiers in the coded data, since this allows user interactions with different instances of the printed medium to be distinguished. This is true whether the coded data directly identifies one or more hyperlinks (or functions), or whether the coded data identifies one or more regions of the medium (or the entire medium). When the coded data includes a coordinate grid, it may include one or more separate identifiers to identify one or more regions of the medium to which the coordinate grid applies, or the coordinate grid may be a fragment of a larger coordinate grid, thus implicitly identifying one or more regions of the medium, e.g. as described in U.S. Pat. No. 6,502,756.
When the coded data is printed onto the medium on demand, there exists the problem of allocating one or more unique identifiers to be included in the coded data. This is particularly a problem when the identifiers are required to be globally unique but the printer does not have online access to a source of globally unique identifiers.